In these days, preparation of a π-conjugated polymer has received much attention due to promising optical and electronic properties, in developing novel organic optoelectronic materials. Research into a mixture of poly(3-hexylthiophene)(P3HT) and [6,6]-phenyl C61 butyric acid methylester (PCBM) used as standard electron donor and electron acceptor materials in bulk hetero junction (BHJ) solar cells (1,2), respectively, has been conducted for the last twenty years, thereby achieving an appreciation regarding Device Physics and improving power conversion efficiency (PCE) to 4 to 5%. However, efficiency of a P3HT/PCBM solar cell is limited by a relatively large band gap of P3HT (˜1.90 eV).
Recently, donor-acceptor of the conjugated copolymer has been extensively used as a donor polymer having a low band ga6pin other organic electronic materials as well as organic solar cells. Significant efforts have been presently focused on finding novel combinations in donor-acceptor units capable of increasing the PCE up to about 10%. Benzothiadiazole (BT) is one of the most promising acceptor units capable of inducing a low band gap. Even though the BT structure has low solubility, several copolymers of electron-abundant units (donor) and BT capable of improving processability were irradiated, which show high PCE at the maximum of 6.1%. Recently, it has been reported that a series of alternate π-conjugated copolymers consisting of BT and different number of hexylthiophene (HT) units are synthesized and characterized, wherein introduction of BT into the poly-HT chain has a significant influence on processability as well as optico-physical and electrochemical properties. Therefore, a co-monomer of 4,7-bis(3,3′/4,4′-hexylthiophene-2-yl)benzo[c][2,1,3]thiadiazole(HT-BT-HT) has received attention as a building block for constituting a low band gap copolymer having a high degree of processability.
Meanwhile, in new organic conductive material fields, ethylenedioxythiophene (EDOT) has been used as a building block in several conjugated systems having combined unique properties such as electrochromic behavior, and the like, and low band ga6ppolymers. A simple, effective, and position-selective synthetic method is required for a bonding of the building block. An arylation method including a metal-catalyst cross coupling reaction of organic metals and organic halides is generally used. Direct and position-selective arylation of thiophene having electron-inducing groups at reactive 2- and/or 5-positions has been recently reported [(a) C. Gozzi, L. Lavenot, K. IIg, V. Penalva, M. Lemaire, Tetrahedron Lett. 1997, 38, 8867-8870; (b) T. Okazawa, T. Satoh, M. Miura, M. Nomura J. Am. Chem. Soc. 2002, 124, 5286-5287; (c) B. Glover, K. A. Harvey, B. Liu, M. J. Sharp, M. F. Timoschenko, Org. Lett. 2003, 5, 301-304; (d) A. Yokooji, T. Satoh, M. Miura, M. Nomura, Tetrahedron 2004, 60, 6757-6763].
In addition, in thiophene having an electron withdrawing group including a methoxy group, aryl halide/heteroaryl halide may also be directly bound at 2-position and/or 5-position under ‘Heck-type’ experiment condition (Jeffery condition). EDPT is basically regarded as 3,4-disubstituted alkoxy thiophene, similar reactivity during an arylation reaction may be shown at 2-position and 5-position to obtain 2-substituted EDOT derivatives. A range of the reaction was successfully extended to synthesize various biaryl compounds in reasonable yields through direct and position-selective C—H arylation of EDOT under experimental conditions of ‘Heck-type’. A copolymer based on EDOT may be favorably prepared by using the above-described developed methods, and thus conventional and general cross-coupling method (Kumada, Negishi, Stille, and Suzuki-type) is avoided.
In addition to an interest in synthesis of oligo- and polythiophene, in order to find a π-conjugated copolymer having a low band gap which is appropriate for a photovoltaic cell and optoelectron, several π-conjugated copolymers are prepared by binding HT-BT-HT co-monomers to other donors and/or acceptors such as 3,4-ethylenedioxythiophese (EDOT), bis-EDOT and thieno[3,4-b]pyrazine (TP) and using general a palladium-catalyst Stille cross-coupling method. Further, several EDOT-based copolymers were designed and successively prepared by direct CH-arylation of the EDOT derivative using various heteroaryl dibromide under ‘Heck-type’ experimental conditions, and compared with the same copolymers prepared by the Stille cross-coupling method. It is shown that positions of hexyl chains in thiophene rings having HT-BT-HT units have important influence on optophysical and electrochemical properties of the polymers.